An apparatus used for application of a sprayed concrete layer to a surface comprises a concrete feed pipe for a pumpable and sprayable concrete mixture made up with water, a sprayer and a compressed air feeder connected to a compressed air source.
The compressed-air-project metering device is under a pressure of several bar. The concrete feed pipe is connected to and opens into the sprayer which is also supplied with the compressed air. The concrete mixture is sprayed using the compressed air as a dispersing and entrainment medium.
The known apparatus includes a compressed air feed device which includes a pipelike duct extending to the air regulating, metering and valve devices, which is connected to the compressed air source and connects and opens into the sprayer. The compressed air is provided at a feed pressure of several bar, e.g. from 5 to 8 bar, which is the static pressure of the compressed air source.
In this way the sprayed concrete mixture can be effectively emplaced, chiefly by a sprayer lance, head or hose.
The compressed air can be relieved (expanded) in the sprayer and, to a greater degree, on issuing from the spray lance or head or in or upon the emergence from the sprayer hose.
For example the sprayer can comprise for example a linear guide pipe section for the compressed air and a pipe connector inclined to that pipe section opening into it for the concrete mixture.
We have found that it is possible to mix the sprayed concrete mixture with a powdery concrete additive. This helps avoid an undesirably high moisture content in the freshly made concrete layer. As a result, we can improve the application of a concrete layer to a surface substantially by directly mixing with the concrete, silica powder immediately prior to spraying or during spraying.
The silica powder eagerly absorbs excess water from the sprayed concrete mixture and allows application of a substantially thicker sprayed concrete layer.
By silica powder we mean amorphous or colloidal silicic acid (colloidal silica) which is made by high temperature hydrolysis or colloidal silica precipitation process.
The silica powder has a very large surface area, e.g. 25 m.sup.2 /g or substantially more. Usually about 1 to 10 weight percent of silica powder relative to the hydraulic cement component of the concrete mix is used. Practical considerations often demand that other powdery concrete additives be used in addition or alternatively.